1. Field of the Invention
The present invention relates to functionalized carbon nanostructures and to their preparation.
2. Description of the Related Art
Carbon nanotubes and fullerene materials are the subject of much current interest. Such materials are not soluble in aqueous, organic or hydrocarbon solvents so efforts have been made to functionalize them to render them soluble in one or more solvent categories.
U.S. Pat. No. 6,187,823 is directed to solubilizing single-walled carbon nanotubes by direct reaction with amines and alkylaryl amines having an uninterrupted carbon chain of at least 5 and preferably 9 carbon atoms in length. The single-walled carbon nanotubes are terminated with carboxylic acid groups, then the carboxylic acid groups are reacted with an amine such as nonylamine or octadecylamine or an alkylaryl amine such as 4-pentylaniline or 4-tetracontylaniline, in an appropriate solvent such as toluene, chlorobenzene, dichlorobenzene, dimethylformamide (DMF) heramethylphosphoramide, dimethylsulfoxide (DMSO) with heating at between 50 to 200° C.
U.S. Published Application U.S. 2003/0065206 is directed to derivatization and solubilization of insoluble classes of carbon nanomaterials which include fullerenes, including very high molecular weight fullerenic materials generated in fullerenic soot, giant fullerenes, fullerenic polymers, carbon nanotubes and metal-carbon nanoencapsulates. The method involves cyclopropanation of the exterior surface of the fullerene or carbon nanotubes. The derivatives formed are described as exhibiting increased solubility in solvents to commonly employed, e.g., non-polar hydrocarbons and arene solvents. The cyclopropanation reaction can be performed on fullerenes or carbon nanotubes, the surfaces of which are devoid of any prior functionalization or on fullerenes or carbon nanotubes which have been previously functionalized yet remain insoluble in solvents. The process involves the cyclopropanation reaction as previously applied to soluble fullerenes by Bingel et al. which involves base-induced deprotination of alpha halo substituted bis-malonates, see e.g. U.S. Pat. No. 5,739,376. The nucleophilic carbanion adds to the fullerene or carbon nanotubes surface, making a new carbon-carbon bond, followed by elimination of the halide ion, completing the cyclopropanation and leaving a derivative group positioned 1, 2 across a carbon-carbon double bond of the fullerene or carbon nanotubes. The reaction is carried out in a heterogeneous mixture in a polar aprotic solvent, e.g. ether, tetrahydrofuran, 1,4-dioxane, dimethoxy-ethane or miscible mixtures thereof. The method is reported as being rapid, does not require heating and does not require the use of strongly coordinating and reactive bases such as amine DBU, but the use of sub-stoichiometric levels of soluble bases such as nitrogen bases and DBU in the presence of an excess quantity of a proton scavenger is also disclosed.
Various cyclopropanation reagents are described, including:
wherein A is a carbon or silicon atom;                LG is the leaving group which includes —Cl, —Br, —I, —OSO2R where R is an optionally substituted alkyl or aryl group;        R1 and R2 are independently selected from the group consisting of optionally-substituted alkyl, alkenyl, alkynyl or aryl groups, —COOR3 groups, —O—CO—R3 groups, —COR3 groups, —CO—NR3R4 groups, —O—CO—NR3R4 groups, —CN, —PO(OR3)(OR4) groups, and —SO2R3 groups wherein R3 and R4 are independently selected from hydrogen, an alkyl group, alkenyl group, alkynyl group or aryl group, any one of which may be optionally further substituted. Preferably R1 and R2 are both —COOR3 groups.        
At paragraph [0134] a sample of single-walled nanotubes was reacted under modified Bingel-type conditions with diethylbromomalonate. It is stated that the derivatization protocol works on single-walled nanotubes, multi-walled nanotubes, nanotubes of varied diameter and both natural length and chemically-shortened nanotubes.
Derivatization renders the derivatized species soluble in common non-polar solvents. Solubility is defined as the dissolution of free molecules (or salts) in the solvent with reversibility to remove the solvent to recover the dissolved molecules or salts. Non-polar solvents are identified as including non-polar organic solvents such as hydrocarbons, and arenes and halogenated arenes, including toluene and benzene.
U.S. Pat. No. 5,739,376 is directed to fullerene derivatives, methods of preparing derivatized fullerenes and methods of using derivatized fullerenes. The fullerene is derivatized using materials of the formula:
    wherein E1 and E2 are identical or different and are each COOH, COOR, CONRR′, CHO, COR, CN, P(O)(OR2) and SO2R where R and R′ are each a straight-chain or branched aliphatic radical (C1 to C20) which may be unsubstituted, monosubstituted or polysubstituted, and X is —Cl, —Br, —I, —OSR2Ar, —OSO2CF3, —OSO2C4F9.
The cyclopropanation reaction is carried out in a base such as alkali metal hydride, alkali metal hydroxide, alkoxide, amide, amine, guanidine at from −78 to 180° C.
The final product can be made directly by using a material:
    wherein E1 and E2 are already in their final desired form or intermediate cyclopropanated fullerenes wherein E1 and E2 are esters can be saponified to give E1 to E2 as corresponding acids, or wherein E1 and E2 are alcohols which are reacted with an acid to give esters of the desired carbon number.
U.S. Published Application U.S. 2006/0210466 is directed to the production of functionalized nanotubes using microwave radiation. The nanotubes material is combined with the functionalizing reactant such as an acid, base, urea, alcohol, organic solvent, benzene, acetone or any other reactant that achieves the desired functionalization reaction, then the mixture is subjected to appropriate microwave conditions to affect the desired functionalization.
“Retention of Intrinsic Electronic Properties of Soluble Single-Walled Carbon Nanotubes after a Significant Degree of Sidewall Functionalization by the Bingel Reaction”, Tomohazu Umegama, et al., J. Phys. Chem. C 2007, 111, 9734-9741 reports single-walled carbon nanotubes functionalized at tips and defect sites with multiple alkyl-substituents and on sidewalls with phenyl-substituents to give sufficient solubility to the nanotube derivatives in organic solvents. Sidewall functionalization utilized the Bingel reaction. This article also reports the shortening of single-walled nanotubes using treatment with HCl and HNO3 aqueous solutions, leaving shortened single-walled carbon nanotubes with carboxylic groups at the upper ends (or tips) and at surface defect sites. These can be reacted with amine materials to yield amide functionalized single-walled carbon nanotubes exhibiting improved dispersibility in common organic solvents such as chloroform, orthodichlorobenzene, tetrahydrofuran.
“Functionalization of Individual Ultra-Short Single-Walled Carbon Nanotubes”, Jared M. Ashcroft, et al., Nanotechnology 17 (2006), 5033-5037 reports the functionalization of 20-80 nm length single-walled carbon nanotubes via in-situ Bingel cyclopropanation. The single-walled carbon nanotubes are shortened via fluorination followed by pyrolysis which both shortens the nanotubes and creates sidewall defects through which various agents can be internally loaded. The shortened single-walled carbon nanotubes are functionalized via the Bingel reaction using a bromomalonate and sodium hydride (NaH) or the Bingel-Hersch reaction using CBr4 and DBU.
“Modification of Multi-Walled Carbon Nanotubes with Fatty Acids and Their Tribological Properties as Lubricant Additives”, C. S. Chen, et al., Carbon 43 (2005), 1660-1666 teaches the treatment of multi-walled carbon nanotubes with a mixture of sulfuric acid and nitric acid to produce an oxidized material which was then boiled in HCl for two hours. The oxidized material was mechanically milled, then sonically mixed with stearic acid in deionized water to which was added sulfuric acid with additionally refluxing at 100° C. for two hours. The reaction mixture was cooled, then extracted with chloroform. Ball milled oxidized multi-walled carbon nanotubes and balled-milled stearic acid modified oxidized multi-walled carbon nanotubes were dispersed in pure liquid paraffin through sonication and stirring. Friction and wear tests were performed. The liquid paraffin containing the stearic acid modified multi-walled nanotubes presented lower friction coefficient and wear loss than did the pure liquid paraffin or the liquid paraffin-containing just the ball milled oxidized multi-walled nanotubes. Wear loss and friction coefficient decreased with increasing mass rates of stearic acid to oxidized multi-walled nanotubes up to a mass ratio of 2.Beyond 2, the friction coefficient and wear loss increased.
“Functionalization of Single-Walled Carbon Nanotubes via the Bingel Reaction”, Karl S. Coleman, et al., J. Am. Chem. Soc. 2003, 125, 8722-8723 teaches the cyclopropanation of single-walled carbon nanotubes. Single-walled carbon nanotubes were annealed under vacuum at 1000° C. for three hours to remove any carboxylic acid groups present on the surface. The decarboxylated single-walled carbon nanotubes were suspended in dry orthodichlorobenzene (ODCB) to which was added diethyl bromomalonate and 1,8-diazabicyclo[5.4.0]undecene (DBU). The mixture was reacted with stirring for two hours and a modified single-wall nanotubes material bearing >C(COO Et)2 groups on the sidewall was isolated. This material was then either trans-esterified with 2-(methylthio) ethanol in diethyl ether and further contacted with a gold colloid to produce functionalized single-walled carbon nanotubes with gold attached to the functional group, or the material was trans-esterified with the sodium or lithium salt of 1H,1H,2H,2H-perfluoro decan-1-ol. These reactions resulted in the introduction of chemical markers into the single-walled carbon nanotubes to facilitate atomic force microscopy visualization and 19F NMR and XPS spectroscopy for surface characterization.